1. Field of the Invention
The present invention relates to a pressurized fluid operated shifting mechanism, which may be (i) of the "X-Y type" for cooperation with a shift bar housing assembly for selectively shifting a change gear mechanical transmission or (ii) of the type utilizing separate cylinder/piston assemblies for each shift fork. In particular, the present invention relates to an automatically or semi-automatically controlled, pressurized fluid actuated shifting mechanism which is effective to utilize dual pressures to improve shift quality and to minimize wear on the shift mechanism components.
2. Description of the Prior Art
Shift bar housing assemblies for mechanical change gear transmissions comprising a plurality of generally parallel, independently axially movable shift bars or shift rails, each carrying a shift fork fixed thereto, and shift block mechanisms allowing a single shift bar to be selected and axially moved to effect engagement/disengagement of a particular gear ratio are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 2,951,392; 4,455,883; 4,575,029; 4,567,785 and 4,584,895, the disclosures of all of which are hereby incorporated by reference.
Typically, such shift bar housings are manually controlled and operated by a shift finger fixed to a directly mounted shift lever or to the cross shaft of a remotely controlled shifting mechanism. Interlock mechanisms are usually provided to prevent movement of more than one shift rail at a time from the axially centered or neutral positions thereof.
Automatically controlled pressurized fluid actuated X-Y type shifting mechanisms wherein a shift finger is automatically moved in the X or rail selection direction and then in the Y or gear engagement/disengagement direction are known in the prior art as may be seen by reference to U.S. Pat. No. 2,931,237, the disclosure of which is hereby incorporated by reference.
Shift bar housing assemblies utilizing pressurized fluid actuated pistons and the like to control each shift rail in an automatic or semi-automatic mechanical transmission are known in the prior art as may be seen by reference to U.S. Pat. Nos. 4,445,393 and 4,722,237, the disclosures of which are hereby incorporated by reference.
The prior art pressurized fluid actuated shifting mechanisms for providing automatic and/or semi-automatic shifting of mechanical change gear transmissions normally utilized with direct and/or remote manual shifting mechanisms were not totally satisfactory as pressurized fluid was supplied either selectively or constantly to selected chambers at only a single relatively high pressure which resulted in certain operations occurring on a more rapid, harsher basis than desired, overshooting of a selected position and/or the possibility of undue wear to components such as shift forks or the like.